Generally, a circular cylindrical shaft which is subjected to a torque is influenced by a pure shear stress. This stress state can be expressed, in terms of its principal stresses, as a compressive stress and a tensile stress, directed perpendicularly thereto, of the same magnitude. The principal stress directions are inclined at .+-.45.degree. to a generatrix to the cylinder surface.
The most commonly used torque measurement which makes use of this is designed such that, within a measurement range of the shaft, a rotationally symmetrical, homogeneous magnetizing field, that is, an H-field, is created with the aid of a surrounding stationary excitation winding. This results in an equally homogenous magnetic flux density, that is, a B-field, in the shaft in unloaded state. When the shaft is loaded, the field configuration of the B-field is distorted, which can be detected with the aid of detection windings.
The state of the art as regards the constructive design of torque transducers based on the above method is disclosed in a number of patent specifications and technical articles. Common to most of these solutions is that two zones are created in the magnetic material, with some type of anisotropy, which causes the magnetic flux density to be deflected at an angle away from its natural direction in parallel with the generatrices to the cylinder surface of the transducer shaft.
According to the state of the art, there are a plurality of different ways of designing the magnetization and detection circuits and of achieving the above-mentioned anisotropy, some of which will be described below.
SU 667836 describes a method in which the anisotropy is created purely geometrically in each zone by cutting grooves in the surface of the shaft according to a specific pattern. This pattern consists of a number of mutually parallel lines directed at an angle of 45.degree. to a generatrix to the cylinder surface of the transducer shaft.
U.S. Pat. No. 4,823,620 describes the same embodiment as above with respect to the geometrical anisotropy, however with the addition that the surface of the shaft is hardened or carburized for the purpose of reducing the hysteresis in the transducer.
Another magnetoelastic method for measuring the torque in a shaft is clear from EP 0 525 551 A2. The shaft whose torque is to be measured is provided with a circularly polarized magnetic ring which is shrunk or glued onto the shaft. When the shaft is loaded, also the ring will be distorted. This means that the peripheral magnetization is changed into a helical orientation with both a peripheral and an axial component.
With the aid of a Hall element which is freely mounted relative to the shaft and which is oriented such that the Hall element only senses the axial component, a measure of the torque occurring in the shaft is obtained.
Both of the above methods entail certain limitations and problems. The first-mentioned method presupposes full rotational symmetry and the shaft must be worked in order to achieve the desired anisotropy. Using a Hall element according to the second method described above also entails several disadvantages. On the one hand, a Hall element will measure the field at one point only along the circumference of the ring. When the transducer shaft rotates, and hence also the circularly polarized magnetic ring, the output signal from the Hall element will vary unless the magnetic field of the ring has a perfect symmetry. Another disadvantage is that Hall elements have very small signal levels. This makes the signal sensitive to electrical interference. In addition, Hall elements have a temperature drift which is not unessential.